A multi-layer printed circuit board used for a high frequency circuit is, for example, disclosed in Japanese Patent Application Publication No. H01-120095 (i.e., U.S. Pat. No. 4,931,354). The multi-layer printed circuit board includes multiple insulation layers made of ceramics and multiple wiring layers having conductivity, which are laminated and integrated together. A cavity is formed between an upper surface and/or a sidewall of an inner conductive wiring layer and the insulation layer disposed on the inner conductive wiring layer.
Since the multi-layer printed circuit board includes the cavity formed between the upper surface and/or the sidewall of the inner conductive wiring layer and the insulation layer disposed on the inner conductive wiring layer, the total dielectric constant of the multi-layer printed circuit board is reduced. Therefore, high frequency properties of the multi-layer printed circuit board such as a delay of propagation of a signal are improved.
However, not only the dielectric constant of the insulation layer but also the surface roughness of the wiring layer affect the high frequency properties of the signal. Thus, the factor for affecting the high frequency properties of the signal is provided by the dielectric constant of the insulation layer and the surface roughness of the wiring layer. As the surface of the wiring layer becomes rough, the surface resistance of the wiring layer becomes larger. Therefore, the properties of a high frequency alternating current flowing through the wiring layer are deteriorated. Specifically, as the frequency of the high frequency alternating current becomes higher, the high frequency alternating current flows nearer the surface of the wiring layer because of a skin effect. Thus, the surface roughness of the wiring layer affects the high frequency properties largely.
Further, another multi-layer printed circuit board is, for example, disclosed in Japanese Patent Application Publication No. 2000-38464 (i.e., U.S. Pat. No. 6,228,467). The multi-layer printed circuit board includes an insulation layer and a wiring layer formed of a conductive pattern, which are laminated together. The multi-layer printed circuit board is manufactured as follows. Multiple conductive pattern films are prepared. The conductive pattern film includes a resin film made of thermoplastic resin and a conductive pattern made of copper foil. The conductive pattern is disposed on the resin film. These conductive pattern films are laminated in a predetermined order, and then, the laminated conductive pattern films are heated and pressurized at a predetermined temperature under a predetermined pressure. Thus, the resin films of the neighboring conductive pattern films are adhered and integrated together, so that the multi-layer printed circuit board is manufactured.
Since all multiple laminated conductive pattern films are adhered at the same time by heating and pressurizing, a process for forming the multi-layer printed circuit board is shortened. Thus, the manufacturing cost becomes small.
However, when the surface roughness of the conductive pattern becomes small in order to improve the high frequency properties, the conductive pattern exposed on the surface of the multi-layer printed circuit board easily peels off.